SEOUL, South Korea (AP)  A huge mushroom cloud that reportedly billowed up from North Korea was not caused by a nuclear explosion, South Korean and U.S. officials said Sunday, but they said the cause was a mystery.

Secretary of State Colin Powell leaves NBC studios, Sunday.

By Manuel Balce Ceneta, AP

The United States has received indications North Korea might be trying to test a nuclear weapon, a senior Bush administration official said Sunday. But the official said there is no evidence that the reported mushroom cloud was linked to the communist nation's suspected nuclear weapons program.

"We're watching the indicators to see whether this is normal activity or whether something else is under way," the official said on condition of anonymity.

Secretary of State Colin Powell confirmed that unusual activity had recently been detected at some of North Korea's atomic sites, but said there was no concrete evidence the North's secretive communist regime was preparing for its first nuclear test explosion.

The South Korean news agency Yonhap reported Sunday that a mammoth explosion in North Korea produced a mushroom cloud more than 2 miles across Thursday. It said the blast was stronger than an April explosion that killed 160 people and injured an estimated 1,300 at a North Korean railway station when a train carrying oil and chemicals apparently hit power lines. (Related story: South Korea reacts to blast)

"There was no indication that was a nuclear event of any kind," Powell said of Thursday's incident. "Exactly what it was, we're not sure."

Kim Jong-min, spokesman for the South Korean presidential office, told Yonhap: "Currently, we are trying to find out in detail the exact character, cause and size of the accident, but we don't think North Korea conducted a nuclear test."

China's government, which has the closest relations with North Korea, had no immediate comment about the reported explosion.

Before Yonhap's report, The New York Times' Sunday editions said senior U.S. intelligence officials had seen signs of activities that some analysts thought might indicate North Korea was preparing a nuclear bomb test. Other experts were more cautious in their assessments, but the developments were considered worrisome enough for the White House to be alerted, the Times said.

Appearing on ABC's This Week, Powell said there were "some activities taking place at some sites that we are watching carefully, but it is not conclusive that they're moving toward a test or they're just doing some maintenance at that site."

The North Koreans "know this would not be a sensible step for them to take," he said. "And it is not just the reaction that they might see in the United States; it's their own neighbors."

On Saturday, North Korea said recent revelations that South Korea conducted secret nuclear experiments involving uranium and plutonium made the communist state more determined to pursue its own nuclear programs.

The South Korean experiments in 1982 and 2000, which the South said did not reflect an attempt to develop weapons, are likely to further complicate six-nation talks aimed at dismantling North Korea's nuclear development. Another round of talks involving the United States, Russia, Japan, China and the two Koreas is tentatively scheduled this month in Beijing.

Washington is pushing for North Korea to fully disclose all of its nuclear activities and allow outside monitoring before it receives any assistance for its struggling economy. North Korea wants energy aid, an end to economic sanctions and removal from Washington's list of state sponsors of terrorism.

"North Korea is looking for assurances that we're not going to invade it, (that) we have no hostile intent," Powell said. "They're looking for benefits for giving up their nuclear capability and their nuclear infrastructure. And what we're debating is what will it take to give them the assurances they need and what benefits would they expect over the long haul."

But, he said, the United States will not "reward them for doing something they should've have been doing in the first place. So we're into a very intense period of negotiations."

Yonhap said the explosion occurred at 11 a.m. Thursday in Yanggang province near North Korea's border with China.

"We understand that a mushroom-shaped cloud about 3.5 to 4 kilometers (2.1 to 2.5 miles) in diameter was monitored during the explosion," Yonhap quoted an unidentified diplomatic source as saying in Seoul, South Korea's capital.

Yonhap said an unidentified South Korean official reported seismic activity related to two blasts in North Korea at 11 p.m. Wednesday and 1 a.m. Thursday. It said a source in China's capital said the explosion left a crater big enough to be seen by a satellite.

Thursday was the anniversary of North Korea's founding on Sept. 9, 1948 — a date North Korean leader Kim Jong Il uses to stage performances and other events designed to bolster loyalty among his countrymen.

Experts have speculated North Korea might use such a major anniversary to conduct a nuclear-related test, but one analyst said an open-air test, as opposed to one below ground, would be difficult in such a small country.

"It's difficult to say, but it won't be easy for North Korea to conduct a nuclear test without resulting in massive losses of its own people," said Koh Yu-hwan, a North Korea expert in Seoul.

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